Lens



co. UVHUS Patented July I l 1939 Search Room LENS George H. Aklin and Fred E. Altman, Rochester, N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application April 16, 1938, Serial No. 202,485

Claims. (CI. 8857) This invention relates to optical s stems and particularly to highly corrected photograp ic objectives.

It relates specifically to that type of objective having two collective components spaced on oppovided the front surface of the component is proportionally strengthened so that the balance of power between the elements of this component and between the three components is not appreciably disturbed.

site sides of a dispersive component, the rear col- In certain preferred embodiments of the inven- 5 lective component being compound. This type tion involving the above features, there are cerof objective is more completely defined by stating tain additional features relating to the front com- (1) that the front collective component has a ponents. In these embodiments, the front colrelatively strong front surface convex to the inlective component is made meniscus and/or the 10 cident with a radius of curvature about AF dispersive component has a front surface whose where F is the focal length of the objective and radius of curvature is slightly greater than F. has a relatively weak rear surface with a radius An objectivemade according to this invention of curvature usually greater than F and usually has generally improved characteristics over other 16 negative (i. e. concave to the incident light) and lenses working at the same relative aperture. In

(2) that the dispersive component is biconcave the examples shown in the accompany drawwith its rear surface the stronger having a radius ing, there is a high degree of correction for astigof curvature about AF and its front surface havmatism. -If the usual rules of weakening the ining a radius of curvature usually between /ZF dividual compo s throughout to ive Wide 20 and -F. It is possible to make either or both of covering power, is applied slightly, the present 20 these components compound to gain slight iminvention holds the astigmatism within tolerance provements without making any radical changes limits and hence may be considered as supplying in the properties of the objective and hence witheither .improved astigmatism or wider covering out departing from this type of objective as above power without loss in correction for astigmatism.

defined. However, these two components are Furthermore, the invention provides better con- 25 usually simple and it is in the simple case that trol over and hence better correction for spherical the present invention is most useful. aberration than in similar objectives.

The invention is concerned primarily with the F g. 1 d 2 Show tWO Slightly e e t rear compound component and with the spacing embodiments of the invention.

of the components. It has for one of its objects The specifications of the lenses shown are as 30 the providing of a .high aperture objective with follows: wide covering power and more highly corrected Fig-1 F=100 mfor spherical aberration than hitherto. Another object is to provide better correction for astig- Lens Glass Radii spacings matism over at least substantially as wide a field 35 as hitherto. I N 1620 R 1 0 According to the invention, the rear air space 13 1. 21 00 (i. e. that between the dispersive component and 11 azi-2 52 the rear collective component) is less than twice III ND; 11637 2L132 3; Z5 the front air space and is preferably but not necessarily greater than this front air space. W 6: 3;? 00 40 Furthermore, the rear collective component is arranged with its crown element to the front and its cemented surface concave to the incident light. 2 m 5 17-100mm The front surface of this rear element is made Lens Glass Rad spacings 45 relatively strong and for ordinary glasses has a radius of curvature less than F. Of course if an mm. mm extremely high index glass is used in this element, I Np= 1.620 R|=+ 27 s t,=t.'o3 say Nn=1.8, this radius may be as large as 1.5F, n 1321 figjflfig 53-13 but for ND less than 1.65, the rule that it should v=3s.2 R|=+ 24 9 sl=6Iso 50 be less than F holds. The rear surface of this 61, 2 fig: ZZZ-8g rear component is made weak having preferably IV Nn= 1152; R1=l20 1 a radius of curvature whose absolute value is v=58'4 greater than F. This radius may be'slightly less than F without departing from the invention provention and by reference to these examples, the following description will explain the pertinent portions of the theory underlying the invention. Although the rear component is the most important in connection with the present invention, the refractive surfaces will be considered in the order encountered by the incident light. The surfaces and their radii of curvature will be designated R1, R2 R1, the thickness of the elements ti t4 and the spacings S1 and S2 in the usual way and as shown in the drawing.

The front crown may be bent to control the flatness of field which is very sensitive in its response to such bending. In the preferred embodiment wherein this component is meniscus, the spherical aberration also responds to such bending. For a satisfactory field R1 must be held between 23% and 35% F and R2 according to its absolute value must be greater than F. It is possible for R2 to be slightly less than F without departing from the present invention especially when the front component is made compound. It might conceivably also be possible to use a very low index glass in this component and have R1 less than 23% F but such cases are undesirable. However, except for the added feature of having this component meniscus in the preferred embodiment, the present invention isnot directly concerned with limitations regarding this front component except to the degree in which they define the class of objectives to which the invention belongs.

This latter statement is also true with regard to the dispersive element wherein the novel feature of the preferred embodiment is that R3 F. R4 is held at about %F.

In an objective made according to the invention and working at f/3.5, S2 must be made less than twice S1, or the distortion (barrel type) will go beyond tolerance limits. In general S2 will be greater than $1, but this is not necessarily true since it is possible to have a lens according to the present invention in which Si is slightly the greater.

As shown in the drawing R5 is between .7F and F, i. e. is less than F. This feature which is quite different from former lenses of this type is a relative one determined by the resulting power of the surface. For all ordinary crown glasses R5 must be less than F but if a glass having an index of refraction Nn 1.8 say, is used, R5 may be slightly greater than F and still meet the requirment of being relatively strong. In such cases where ND is greater than 1.65, R5 must be less than 2F (for all known glasses) and may be less than F.

Although R5 according to the invention is relatively strong compared to R5 in other lenses of this type, it is the outer surface of the crown element of the rear component and corresponds to R7 of lenses wherein the crown element is placed to the rear. This R1 in known lenses is usually strong being less than /gF. Thus the invention has the added advantage of providing relatively weak outer surfaces on both sides of the rear component R5 being greater than .6F' and R1 being in the examples shown greater than F (negative). It is even possible within the invention to make R1 plano i. e. infinite. On the other hand it is possible'to allow R1 to be slightly less than F depending on the glasses used and the covering power desired and hence it is not easy to define the limits for R1 except that it is always greater than F.

The cemented-surface Rs concave to the incident light has been found to control the spherical aberration of the system and must be held at about F (negative)-i. e. between 15% and 35% F. It is curious to note that similar absolute limitations apply to known lenses in which this surface is pgsitive.

Regarding the distribution of powers between the components, in Fig. 1 the focal lengths of the front, dispersive, and rear components are respectively 51, 32 and 56 mm. approximately and in Fig. 2 are respectively 50, 32 and 58 mm. approximately. weakening these powers by changing the radii and/or the glass to say 52, -34 and 60 gives greater covering power but slightly less correction for astigmatism and other aberrations. To avoid lateral color and distortion, this balance of powers must be maintained and satisfactory results are impossible if too much power is shifted from the rear component to the front i. e. if the focal length of the rear component is weakened to say '75 (i. e. %F); it is preferably less than 65 (i. e. .65F). true of all lenses of this type working at an aperture of f/3.5 or larger as is the fact that the front component has usually about twice the power of the whole objective (i. e. its focal length is between .751 and AF) When a lens is made up according to the invention. the rear crown element must have a thickness 1.: greater than 4.5% F to permit an aperture of f/3.5 and practically this ta must be greater than 5% to give any useful thickness at the margins of the lens. In the examples shown is is 8% F.

Having thus described two embodiments of the invention in detail, and the limitations some of which are necessary and some of which are incidental to the invention, we wish to point out that the invention is not limited to these specific structures but is of the scope of the appended claims.

We claim:

1. An objective comprising two collective components axially spaced on opposite sides of a dis-- persive component, the front collective component having about twice the power of the whole objective and the rear collective component having power at least one third greater than and less than three times that of the objective, the front surface of the front component and the rear surface of the dispersive component being convex to the incident light and having radii of curvature each equal to about one fourth of the focal length of the objective, the rear surface of the front component having a radius of curvature according to its absolute value greater than one half of the focal length of the objective, the front surface of the dispersive component being concave to the incident light with a radius of curvature greater than one half of the focal length of the objective, and the rear collective component bein com said objective being characterized by the spacing between the rear component and the dispersive component being less than twice and more than three fourths the other spacing, the cemented surface of the rear component being concave to the incident light, the rear surface of the rear component having a radius of curvature greater than one half of the focal length of the objective and the front surface of the rear component being convex to the incident light with a radius of curvature between" 60% of and twice the focal length of the objective for all glasses and less than this focal length when a crown glass having an index of This is 88. OPTICS refraction with respect to the D line less than 1.65 is used for this front element of this rear component.

2. An objective according to claim 1 in which the front component and the dispersive component are single elements.

3. An objective comprising two collective components axially spaced on opposite sides of a dispersive component, the front collective component having about twice the power of the whole objective and the rear collective component having power at least one third greater than and less than three times that of the objective, the front surface of the front component and the rear surface of the dispersive component being convex to the incident light and having radii of curvature each equal to about one fourth of the focal length of the objective, the rear surface of the front component having a radius of curvature according to its absolute value greater than one half of the focal length of the objective, the front surface of the dispersive component being concave to the incident light with a radius of curvature greater than one half of the focal length of the objective, and the rear collective component bein compound, said objective being characterized by the spacing between the rear component and the dispersive component being less than twice and more than once the other spacing, the cemented surface of the rear component being concave to the incident light with a radius of curvature between 15% and of the focal length of the objective, the rear surface of the rear component having a radius of curvature greater than said focal length, the front surface of the rear component being convex to the incident light with a radius of curvature between 70% and 100% of said focal length and the front element of this rear component having a thickness between 4.5% and 12% of the focal length.

4. An objective comprising two collective components axially spaced on opposite sides of a dispersive component, the front collective component having about twice the power of the whole objective and the rear collective component having power at least one third greater than and less than three times that of the objective, the front surface of the front component and the rear surface of the dispersive component being convex to the incident light and having radii Search Room of curvature each equal to about one fourth of the focal length of the objective, the rear surface of the front component having a radius of curvature according to its absolute value greater than one half of the focal length of the objective, the front surface of the dispersive component being concave to the incident light with a radius of curvature greater than one half of the focal length of the objective, and the rear collective component lging compound, said objective being characterized by e spacing between-the rear component and the dispersive component being less than twice and more than three fourths the other spacing, the cemented surface of the rear component being concave to the incident light; each outer surface of the rear component having a radius of curvature greater than one half of the focal length of the objective and the front component of the objective being meniscus with a radius of curvature for its rear surface less than ten times the focal length of the objective.

5. An objective comprising two collective components axially spaced on opposite sides of a dispersive component, the front collective component having about twice the power of the whole objective and the rear collective component having power at least one third greater than and less than three times that of the objective, the front surface of the front component and the rear surface of the dispersive component being convex to the incident light and having radii of curvature each equal to about one fourth of the focal length of the objective, the rear surface of the front component having a radius of curvature according to its absolute value greater than one half of the focal length of the objective, the front surface of the dispersive component being concave to the incident light with a radius of curvature greater than one half of the focal length of the objective, and the rear collective component being compound, said objective being characterized by the spacing between the rear component and the dispersive component being less than twice and more than three fourths the other spacing, the c surface of the rear component be concave to the incident light and the front surface of the dispersive component having a radius of curvature greater than the focal length of the objective.

FRED E. ALTMAN. GEORGE H. AKLIN. 

